The present invention involves a star coupler for directly coupling signals between a first group of optical fibers and a second group of optical fibers.
In recent years there has been a growing interest in the possibility of utilizing optical fibers for the transmission of information. Optical fibers offer the advantages of exhibiting low loss in the amplitudes of the signals and providing a high bandwidth thereby enabling a significant amount of information to be transmitted over each fiber. In order to transmit information between a plurality of terminals through the optical fibers, it is necessary that the fibers be coupled together in some manner; otherwise, there would have to be separate fibers interconnecting each terminal. A Tee coupler system has generally been employed for this purpose. In a Tee type coupler, the optical fibers from a terminal are coupled to a general transmission line which then carries the signals to other terminals. Such Tee type couplers, however, suffer from connector losses and packing fraction losses.
In order to overcome some of the problems involved with the Tee type couplers, a star type coupler has been developed by Corning Glass Works. The star coupler transfers signals transmitted along optical fibers from a first group of terminals to the optical fibers coupled to a second group of terminals. In such a star type coupler, both the transmitting and receiving fiber optics are connected to a cladded glass mixing rod. The fibers can be either supplied at one end of the mixing rod with a mirror being positioned at the opposite end or the fibers from one set of terminals can be supplied to one end of the mixing rod while the fibers from the other group of terminals are supplied to the opposite side. In either case, however, the fibers do not pass through the mixing rod. Instead, as the signals leave the ends of the fibers and pass through the mixing rod, the signals are internally reflected by the glass core of the mixing rod. By providing a mixing rod of an appropriate length, sufficient reflections within the rod occur in order that the signals supplied by any of the fibers from one group of terminals will be reflected within the rod and received by all of the fibers of the other group of terminals.
Such Tee type couplers and star type couplers are discussed in an article entitled "The Star Coupler: A Unique Interconnection Component for Multimode Optical Waveguide Communications Systems", Applied Optics, Vol. 13, No. 11, November 1974. This article from Applied Optics discloses a star coupler in which all of the fibers enter from one side with a mirror being positioned at the other side.
As previously noted, the fibers from a first group of terminals can enter such a star coupler from one end while the fibers from the other group of terminals are connected to the star coupler at the opposite end. An example of the second type of star coupler is shown in British Pat. No. 1,406,277 to Plessey Company Ltd., which was published on Sept. 17, 1975. In the star coupler disclosed by the British Patent, an uncladded end portion of each fiber of the first group of fibers is fed a short distance into one end of the coupler, while the uncladded end portions of a second group of fibers are fed into the coupler from the opposite end. By providing glass mixing rods within the center of the coupler in a space between the two groups of fibers, the signals from any fiber are reflected by the glass wall of the mixing rod so that such signals are received by all the fibers of the other group. Another embodiment of this second type of a star coupler is illustrated in U.S. Pat. No. 3,937,557 to Milton, which issued on Feb. 10, 1976.
In comparison with the Tee coupler system, the star coupler system has been shown to exhibit a lower terminal-to-terminal loss, easier upgradability and better reliability. Moreover, the star coupler can be constructed with a very low insertion loss and one such coupler can serve a plurality of terminals.
The star coupler employing a mixing rod can be utilized for coupling signals between both fiber bundles and single fiber configurations. This type of star coupler, however, exhibits several disadvantages: high insertion loss (although the loss is less than that of the Tee coupler), uneven light distribution, high fabrication cost and no directivity (i.e. since the coupler is essentially bi-directional the signals which are reflected within the mixing rod can be transmitted back along the fiber which has supplied the signal). With respect to the last problem, it is noted that backtracking of the signal can cause distortion in the signals being transmitted and for this reason, it is preferable for the coupler to be directional.
It has also been known that information can be coupled between two optical fibers by arranging a portion of each fiber in an adjacent parallel relationship. If the adjacent portions of the fibers are free of any cladding, then the signals for one fiber will be transmitted into the adjacent fiber. An example of such a system is illustrated in U.S. Pat. No. 3,809,686 to Chandross et al, which issued May 7, 1974.